Polycyclic aromatic hydrocarbons (PAHs) are persistent, potentially carcinogenic organic contaminants commonly found in soils at industrial, brownfield, and former gasworks sites. This technical article provides a comprehensive overview of site soil PAH analysis, covering sampling strategies, the 16 priority PAH target analytes, extraction and instrumental methods such as GC-MS and HPLC, relevant international and national standards, quality assurance requirements, and typical applications in contaminated site assessment and remediation.
Technical Overview
Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds composed of two or more fused aromatic rings. They are formed primarily through the incomplete combustion of organic materials such as coal, petroleum, wood, and biomass, and are also present in coal tar, creosote, asphalt, and refined petroleum products. Due to their hydrophobicity (high octanol–water partition coefficients) and chemical stability, PAHs strongly adsorb onto soil organic matter and persist in the subsurface environment for years or decades.
Site soil PAH analysis refers to the systematic sampling, extraction, purification, and instrumental determination of PAH concentrations in soil to support contaminated site investigation, human health and ecological risk assessment, remediation design, and regulatory compliance. The United States Environmental Protection Agency (US EPA) has designated 16 PAHs as priority pollutants, and several of them—including benzo[a]pyrene, benz[a]anthracene, and dibenz[a,h]anthracene—are classified as probable or known human carcinogens by the International Agency for Research on Cancer (IARC).
Regulatory frameworks worldwide govern soil PAH screening. In China, GB 36600-2018 (Soil Environmental Quality—Risk Control Standard for Soil Contamination of Development Land) establishes risk screening values for individual PAH compounds differentiated by land-use sensitivity; for example, the screening value for benzo[a]pyrene is 0.55 mg/kg for sensitive land uses and 1.5 mg/kg for less sensitive industrial land uses. Internationally, ISO 18287 and US EPA Methods 8270E and 8310 provide standardized analytical procedures. Accredited third-party laboratories routinely perform these analyses under ISO/IEC 17025 quality systems to ensure data defensibility for regulatory submission.
Testing Samples
Surface Soil Samples
Surface soil (typically 0–50 cm) is collected using stainless steel scoops, trowels, or hand augers from targeted sampling points identified during preliminary site investigation. Surface samples are critical for evaluating direct-contact exposure pathways, dermal contact, and incidental ingestion risks at recreational, residential, and commercial redevelopment sites.
Subsurface Soil and Borehole Samples
Subsurface samples are obtained via drilling methods such as hollow-stem auger, direct-push technology, or rotary drilling. Samples are generally collected at continuous depth intervals or at lithological boundaries and apparent contamination zones (based on visual, olfactory, or photoionization detector screening) to characterize vertical contaminant distribution and delineate the three-dimensional extent of PAH contamination.
Sediment and Groundwater-Adjacent Samples
Riverbank, ditch, and drainage channel sediments adjacent to industrial sites are frequently analyzed because PAHs migrate from soil to sediment compartments. These samples help assess off-site migration and potential ecological risks to aquatic receptors.
Remediation Verification and Stockpile Samples
During and after remediation (e.g., excavation, thermal desorption, bioremediation, or chemical oxidation), confirmation samples from excavation sidewalls and bottoms, treated soil stockpiles, and backfill material are analyzed to verify that residual PAH concentrations meet the applicable cleanup or screening criteria.
Sample Collection, Preservation, and Holding Requirements
- Samples are collected in pre-cleaned amber glass jars with PTFE-lined caps to minimize photodegradation and cross-contamination.
- Samples are cooled and transported at 4 °C or below with chain-of-custody documentation.
- Extraction is generally recommended within 14 days of collection when samples are stored under refrigerated conditions, in accordance with US EPA and HJ method requirements.
- Sufficient sample mass (typically 100–500 g) is collected to allow for moisture determination, repeat analysis, and laboratory QC requirements.
Testing Items
The standard analytical scope covers the 16 US EPA priority PAHs, which are also the target compounds specified in Chinese standards GB 36600-2018 and HJ 783-2016:
- Naphthalene
- Acenaphthylene
- Acenaphthene
- Fluorene
- Phenanthrene
- Anthracene
- Fluoranthene
- Pyrene
- Benz[a]anthracene
- Chrysene
- Benzo[b]fluoranthene
- Benzo[k]fluoranthene
- Benzo[a]pyrene
- Indeno[1,2,3-cd]pyrene
- Dibenz[a,h]anthracene
- Benzo[g,h,i]perylene
In addition to individual compound concentrations, reporting typically includes total PAHs (the sum of the 16 compounds) and the sum of the seven carcinogenic PAHs. For human health risk assessment, benzo[a]pyrene-equivalent concentrations (BaP-TEQ) may be calculated by applying toxic equivalency factors to the carcinogenic PAHs, providing a toxicity-weighted metric for comparison against risk-based criteria.
Method detection limits for routine GC-MS analysis generally range from 0.005 to 0.1 mg/kg on a dry-weight basis, depending on the instrument, sample mass, and cleanup procedure, and are adequate for comparison with most international screening values.
Testing Methods
Sample Preparation
Soil samples are typically air-dried in the dark or freeze-dried to remove moisture without volatilizing low-molecular-weight PAHs. The dried sample is gently disaggregated, sieved (commonly through a 2 mm or 0.25 mm mesh), homogenized, and a subsample is weighed for extraction. Moisture content is determined concurrently so that results can be reported on a dry-weight basis.
Extraction Techniques
- Accelerated solvent extraction (ASE, US EPA Method 3545A) or pressurized liquid extraction: uses elevated temperature and pressure with organic solvent (e.g., acetone/hexane or dichloromethane) for fast, efficient, and automated extraction.
- Soxhlet extraction (US EPA Method 3540C): a classical, exhaustive technique often used as a reference method for heavily contaminated soils.
- Ultrasonic extraction (US EPA Method 3550C): a rapid approach suitable for high-throughput screening.
- Microwave-assisted extraction: provides efficient solvent recovery and reduced extraction times and is specified in several national standard methods.
Cleanup and Purification
Crude extracts often contain co-extracted humic substances, mineral oils, and elemental sulfur that can interfere with chromatographic determination. Cleanup is performed using silica gel, neutral alumina, or Florisil column chromatography, sometimes combined with gel permeation chromatography (GPC) or activated copper for sulfur removal, following procedures such as US EPA Method 3630C.
Instrumental Determination
- Gas chromatography–mass spectrometry (GC-MS, US EPA Method 8270E, ISO 18287, HJ 783-2016): the most widely used technique, operating in selected ion monitoring (SIM) mode for maximum sensitivity and compound-specific confirmation via retention time and ion ratios.
- High-performance liquid chromatography with fluorescence detection (HPLC-FLD, US EPA Method 8310, HJ 805-2016): offers excellent sensitivity and selectivity for PAHs, particularly heavy 4–6 ring compounds, using programmable wavelength switching.
- GC-MS/MS: employed when significantly lower detection limits are required or when complex matrix interferences demand enhanced selectivity.
Quality Assurance and Quality Control
Analyses are conducted under ISO/IEC 17025-accredited quality systems, with QC measures including method blanks, laboratory control samples, matrix spikes, duplicate samples, multi-point calibration with continuing calibration verification, and surrogate standards such as naphthalene-d8, acenaphthene-d10, phenanthrene-d10, chrysene-d12, and perylene-d12. Surrogate recoveries typically within 50–150% and matrix spike recoveries within method-specified criteria are required for data acceptance, and any deviations are qualified and documented in the analytical report.
Applications
Site soil PAH analysis supports a wide range of environmental management and regulatory activities:
- Phase II environmental site assessments and detailed site investigations at former gasworks, coking plants, steel mills, refineries, fuel stations, wood-treatment facilities, and tar or asphalt production sites.
- Brownfield redevelopment and land-transfer due diligence, confirming whether soil quality satisfies GB 36600-2018 screening values or other jurisdictional criteria prior to residential or commercial reuse.
- Human health and ecological risk assessment, providing concentration data for exposure modeling and derivation of site-specific remediation targets.
- Remediation design and verification, including evaluating the effectiveness of excavation, thermal desorption, soil washing, chemical oxidation, and bioremediation, and confirming backfill quality.
- Regulatory compliance monitoring and acceptance by environmental authorities for site closure and contamination removal sign-off.
- Agricultural and urban soil quality surveys, urban parkland and school site evaluation, and roadside soil studies related to traffic emissions.
- Environmental litigation and insurance claim support, where legally defensible, chain-of-custody-documented data are required.
FAQ
Why are PAHs a concern in site soils?
PAHs are persistent, bioaccumulative, and several members of the group are carcinogenic, mutagenic, or teratogenic. Because they bind strongly to soil organic matter and degrade slowly, historical industrial operations can leave elevated PAH residues decades after operations cease, posing long-term risks through direct contact, ingestion, and vapor intrusion of low-molecular-weight compounds.
What is the difference between GC-MS and HPLC methods for PAH analysis?
GC-MS (e.g., US EPA 8270E, HJ 783-2016) provides compound-specific mass spectral confirmation and is well suited to the full suite of 16 PAHs, including volatile naphthalene. HPLC with fluorescence detection (e.g., US EPA 8310, HJ 805-2016) offers very high sensitivity for heavier 4–6 ring PAHs. Both are acceptable regulatory methods, and the choice depends on project requirements, matrix complexity, and detection limit objectives.
How are results reported and evaluated?
Results are reported in mg/kg on a dry-weight basis for each individual PAH, together with total PAHs and, where requested, BaP-toxic equivalents. Concentrations are compared against applicable screening values such as those in GB 36600-2018, US EPA regional screening levels, or site-specific risk-based targets to determine whether further assessment or remediation is required.
What sample holding times and conditions apply?
Soil samples for PAH analysis should be stored in amber glass containers at 4 °C or below and extracted within 14 days of collection in accordance with standard method requirements. The holding clock is stopped once the sample is extracted, and extracts are typically analyzed within 40 days.
How long does laboratory analysis take?
Standard turnaround time for routine PAH analysis is typically 7 to 10 working days from sample receipt, depending on sample load, matrix complexity, and any required dilution or re-extraction. Expedited services of 3 to 5 working days are commonly available from accredited third-party laboratories for time-critical remediation projects.
What factors can affect data quality?
Heterogeneous distribution of PAHs in soil, high moisture content, co-eluting petroleum hydrocarbons, and sulfur interference are common challenges. Thorough sample homogenization, appropriate cleanup procedures, surrogate recovery monitoring, and duplicate sampling are essential to ensure representative and defensible results.